INTRODUCTION

Stock Assessment Report 2024

Stock Assessment Report 2025

Objectives

For Yellowfin, Bigeye, and Skipjack tuna.

  1. Estimate key biological and exploitation reference points (e.g., size at maturity, growth rates, mortality) for Yellowfin, Bigeye, and Skipjack tuna.
  2. Assess the current stock using the Length-Based Spawning Potential Ratio (LBSPR) model.
  3. Identify and map the spatial distribution of fishing intensity

Approach

Theorem- LBSPR Model

Length-Based Spawning Potential Ratio (LBSPR)

\[ \begin{align} SPR &= \frac{\text{Fished}}{\text{Unfished}} \\ &= \frac{\sum_{L} \phi(L) \cdot F(L)}{\sum_{L} \phi_0(L) \cdot F(L)} \end{align} \]

Assumptions

  1. Unit Stock: The populations are treated as single, self-contained stocks for this assessment.
  2. Representative Sampling: The data collected from the artisanal and industrial fisheries are representative.
  3. Equilibrium State: The fishery and the underlying population are assumed to be in a state of equilibrium.
  4. Closed Population: The stocks are considered demographically closed, with negligible immigration or emigration during the assessment period.

Length and Weight Data

Neritic waters

Data from the neritic zone were sourced from both TAFIRI and DSFA

EEZ

Data from Exclusive Economic Zone were sourced from DSFA

Fishing grounds

Sampling months

“The sampling spans all 12 months of 2024, ensuring comprehensive coverage. Data was collected consistently from January through December.”

Length and Weight Data…

Length Frequencies for 2024

Yellowfin tuna

Skipjack tuna

Bigeye tuna

Reference points & Stock

Yellowfin Tuna (Thunnus albacares)

Juvenile and adult

Reference Points

Yield and Biomass

Summary

  • SPR: 29%
    • Indicates the stock is within the recommended range.
    • This is an improvement over the 2024 report’s estimate of 13%, but still well below the 40% target.
  • Critical Issue: Juvenile Catch
    • Maturity Size (\(L_{50}\)): 92.3 cm
    • Capture Size (\(SL_{50}\)): 59.5 cm
    • This large gap shows that fishing pressure is heavily concentrated on immature fish, severely impacting the stock’s ability to replenish itself.

Skipjack Tuna (Katsuwonus pelamis)

Juvenile and adult

Reference Points

Yield and Biomass

Summary

  • SPR: 33% (New Baseline)
    • This is the first formal assessment for Skipjack in this series.
    • The 33% SPR suggests the stock is within the sustainable level.
  • Concern: Juvenile Catch
    • Maturity Size (\(L_{50}\)): 59.9 cm
    • Capture Size (\(SL_{50}\)): 50.2 cm
    • Like the other species, a substantial portion of the catch consists of immature fish that have not had a chance to spawn.

Bigeye Tuna (Thunnus obesus)

Juvenile and adult

Reference Points

Yield and Biomass

Summary

  • SPR: 33%
    • Indicates the stock is within the sustainable fishing level.
    • Represents a significant improvement in estimated status from the 2024 report’s 14% SPR.
  • Concern: Juvenile Catch
    • Maturity Size (\(L_{50}\)): 82.0 cm
    • Capture Size (\(SL_{50}\)): 62.7 cm
    • While less severe than for Yellowfin, the capture of immature individuals remains a key driver of stock pressure.

Key Finding

Overall Key Finding

All three assessed tuna stocks are within the recommended range of sustainable levels.

  • The Spawning Potential Ratio (SPR) for all species is above 20% but below the 40% target reference point.

  • The primary driver is the harvest of immature fish: The size at which fish are caught (\(SL_{50}\)) is consistently and significantly lower than the size at which they mature (\(L_{50}\)).

A Comparative Summary

The consistent gap between size at maturity (\(L_{50}\)) and size at capture (\(SL_{50}\)) is a major red flag.

Species Spawning Potential (SPR) Size at Maturity (\(L_{50}\)) Size at Capture (\(SL_{50}\)) Status
Yellowfin 29% 92.3 cm 59.5 cm Sustainable
Bigeye 33% 82.0 cm 62.7 cm Sustainable
Skipjack 33% 59.9 cm 50.2 cm Sustainable


An SPR range between 20% and 40% is the recommended.

Harvesting Immature Fish

The Core Problem: the average size at capture is significantly smaller than the size at maturity.

Key Sobering Thoughts!!!

What is a key assumption of the LBSPR model?

  • The population is rapidly growing
  • The fishery is in equilibrium
  • Fishing only occurs in deep-sea waters
  • Data is globally applicable

What is the primary concern for tuna stocks?

  • Overfishing adults
  • Insufficient data
  • Catching juvenile fish
  • Environmental factors

What is the sustainable Spawning Potential Ratio (SPR) range?

  • Below 20%
  • 20% to 40%
  • Above 40%
  • Above 10%

How did the Yellowfin SPR change from 2024 to 2025?

  • It decreased significantly
  • It remained the same
  • It improved from 13% to 29%
  • It was not assessed in 2024

For which species is the gap between maturity size (L50) and capture size (SL50) the largest?

  • Yellowfin Tuna
  • Bigeye Tuna
  • Skipjack Tuna
  • The gap is similar for all species

What is the HIGHEST priority management recommendation?

  • Enhance data collection
  • Protect juvenile fish
  • Promote sustainable practices
  • Research migration patterns